Foot grilles are often used at the entrances to public and commercial buildings to catch dirt that people entering would otherwise track into and through the building. The same grilles also can be used as gratings in places such as floor and walkway openings and the filter return overflow troughs around swimming pools.
The assignee of the present invention has for many years manufactured, in accordance with U.S. Pat. No. 3,383,822 issued May 21, 1968, a foot grille which is made from only two major components (plus, in most cases, a tread insert component) and which is very simple to assemble. The use of only two (or three) principal components and the ease of assembly have kept the cost low; the structure of the grille is such that, when properly fitted in a floor recess or opening having a flat rigid base for supporting the grille base throughout or keeping unsupported spans low, the grille stays tight and provides good service for many years.
The grille of U.S. Pat. No. 3,383,822 does lack one desirable property, a high transverse stiffness; the T-shaped tread rails provide considerable lengthwise stiffness, but the slotted lock bars are, structurally, little more than tie rods and, compared to the tread rails, provide only a moderate level of resistance to bending across the width. In some installations, notably those in which there are large unsupported spans, and under severe use over an extended time, the grille deflects and works and eventually loosens.